THE LIFE-CYCLE OF THE INTENSE IOP-14 STORM DURING CASP-II .1. ANALYSIS AND SIMULATIONS

In this study, a 5-day life-cycle of the IOP-14 storm during GASP II i s examined using conventional observations and numerical simulations w ith a mesoscale version of the Canadian Regional Finite-Element (RFE) model. Observational analysis reveals that the IOP-14 storm forms from a lee trough, occurring along a strong baroclinic zone with an intens e frontogenetic deformation, that interacts with all upper-level trave lling short-wave trough across the Canadian Rockies. Then the storm ex periences a slow, but nearly steady, growth while traversing the North American continent. It deepens explosively as it moves into the Atlan tic Ocean. It appears that i) the enhanced large-scale baroclinicity d ue to land-sea temperature contrasts, ii) the tremendous latent heat r elease due to the transport of high-theta(e) air from the marine bound ary layer, iii) the decrease of surface drag and iv) the favourable we stward tilt of the low with an amplifying trough all contribute to the explosive deepening of the storm.Two consecutive simulations covering a total of 102 h during the storm development are carried out with a grid size of 50 km. The RFE model reproduces very well the formation o f the surface low on the lee side of the Rockies, the track and deepen ing rates, the explosive development and decay of the storm, and vario us mesoscale phenomena (e.g., a ''bent-back'' warm front, a ''T-bone'' thermal pattern, a cold frontal ''fracture'', an upper-level ''eye'' and warm-core structures), as verified by conventional observations, s atellite imagery, flight-level and dropsonde data from a research airc raft. It is found from potential vorticity (PV) analysis that the stor m reaches its peak intensity as the upper-level dry PV anomaly, the lo w-level moist PV anomaly and surface thermal warmth are vertically sup erposed. PV inversions reveal that these anomalies contribute about 60 %, 30% and 10%, respectively, to the 900-hPa negative height perturbat ion. It is shown that the warm-core structure near the cyclone centre is produced by advection of warmer air ahead of the cold front, rather than by adiabatic warming associated with subsidence.